Pneumatic control of furnaces



Signal Air June 18, 1968 .1. GABRIELSON 3,388,862

PNEUMATIC CONTROL OF FURNACES Filed Dec. 1, 1965 5 Sheets-Sheet 1 FIGUREI Burner Analog 7 Controller 8 Fuel Combustion Air H F Burner lo MBurner Fuel Fuel L Signal Air ,Pneumclic Valve Comkpsflon FIGURE 20JAMES E. GABRIELSON mvsmoa .Y aa/ PATENT ATTORNEY June 18, 1968 J. E.GABRIELSON Filed Dec. 1, 1965 FIGURE 2b Pneumallc Valve 3 Sheets-Sheet 2FIGURE 20 Pneumatic Valve Burnev Air Signal 5 g Fluid Pneumallc Valve 33Fuel INVENTOR PATENT ATTORNEY JAN 8 E. GABRIELSON June 18, 19683,388,862

J. E. GABRIELSON PNEUMATIC CONTROL OF FURNACES Filed Dec. 1. 1965 5Sheets-Sheet 5 Fuel JAMES E. GABRIELSON mvzuron FIGURE 4 PATENT ATTORNEYCombustion Air United States Patent ABSTRACT OF THE DISCLOSURE A methodand apparatus for controlling the fuel rate or combustion air rate to aburner. A Y-shaped flow diverter valve is controlled by a constantvolume pump supplied stream of control fluid which passes in a conduitthrough the burner flame. More or less combustion air or fuel isdirected by the diverter valve through the burner in response to changesin energy level of the heated control fluid.

This invention relates to multiple-burner fluid hydrocarbon fuel firedfurnaces. In particular, it rel-ates to the pneumatic control of the airflow to burners in order to maintain the optimum level of excess air atthe burners.

Multiple-burner residual fuel fired furnaces or boilers are commonlyused in firing power plants. Such furnaces or boilers may have a dozenburners, be as tall as an eight story building, and consume up to 400barrels of fuel per hour. The residual fuel oils employed in suchfurnaces contain sulfur and metallic contaminants, both combustible andnoncombustible, that form ash upon the combustion of the fuel. Theformation of ash as a result of the combustion of the residual fuelresults in deposit formations or slagging which reduces heat transferefliciency. The sulfur in the fuel also causes problems. Some heavyfuels derived from high sulfur crudes contain as much as from 1 to 5 wt.percent sulfur. The burning of the sulfur-containing fuels results inthe formation of sulfur trioxide which promotes corrosion in thesuperheaters and preheaters.

It is Well known that the operation of residual fuel fired furnaces canbe substantially improved in terms of inhibiting the amounts of sulfurtrioxide formation by carrying out the combustion at levels of lowexcess air that are only slightly higher than stoichiometric; forexample, from 0.01 to 5% excess. The use of higher amounts, for example,to excess air results in the formation of the highly corrosive sulfurtrioxide. The use of low excess air is an effective method of inhibitingcorrosion, but it must be carefully controlled because the use of air inless than stoichiometric amounts results in smoke formation and poorcombustion. In short, at least a stoichiometric amount of air must beused in order to avoid excessive smoke production and not more than 5%excess air can be used or excessive amounts of sulfur trioxide areproduced.

Even though it is well recognized that burners in such furnaces orboilers should be operated at slightly more than stoichiometric and lessthan 5% excess air, one still finds that there are problems incontrolling the system to achieve the optimum operation. If only one ortwo burners out of twelve are not operating within the proper range,either excessive amounts of smoke or sulfur trioxide will be produced.Thus, it is preferable to have all the burners in the furnace operatingat either end of the optimum range rather than having ten or elevenoperating in the middle of the range and one or two operating outside ofthe range. It is difficult to maintain the proper amount of excess airat each burner, because conditions may change for one or more burnersduring operation. For example, the nozzle of one burner may wear or the3,388,852 Patented June 18, 1968 quality, inlet temperature, or rate ofthe fuel to an individual burner may change during operations.

It is therefore an object of this invention to provide a system forproportioning the combustion air so that each burner is operating withinthe optimum range of excess air.

Other objects of this invention will become apparent as one reads thefollowing description in connection with the attached drawings in which:

FIGURE 1 schematically depicts a control system for single burner.

FIGURES 2a, 2b, and 2c are cross-sectional views of the control valveemployed by the invention.

FIGURE 3 depicts of a prop-ortioning system wherein fuel rate is themanipulated variable.

FIGURE 4 is a schematic diagram of a bank of burners wherein thecombustion air is proportioned in accordance with the present invention.

Flame temperature is an indication of the amount of excess air providedto each burner. If flame temperature is too high, the burner is notreceiving enough excess air. Thus, if the flame temperature goes abovethe desired range, the flow of combustion air to the burner can beincreased or the fuel rate decreased to return the system to optimumconditions. Various temperature measuring devices can be employed todetermine flame temperatures such as thermocouples, photopyrometers,ultraviolet analyzers, and microwave analyzers. These devices can beadapted to give an accurate determination of flame temperature. However,many of the devices, along with the associated control equipment, arecomplicated and expensive to install as well as expensive to maintain.

In accordance with this invention, a change in flame temperature isdetected by a stream of control fluid, for example, air, and theresulting change in energy of the control fluid initiates correctiveaction either to the combustion air flow rate or the fuel rate. In oneembodiment of this invention, there is a control loop for proportioningexcess air between burners which has two small pumps as the only movingparts.

In each embodiment of the present invention a control fluid, forexample, air, nitrogen, or other suitable fluid, is heated by the flameof a burner to be controlled. The heat from the flame increases theenergy of the control fluid thereby increasing the temperature andincreasing the momentum of the control fluid. The change in momentum ofthe stream is used to initiate a change in either combustion air or fuelto the burner.

In the first embodiment of this invention, the control fluid actuates ananalog controller which in turn adjusts the flow of combustion air orfuel. In a preferred embodiment of this invention, combustion air isproportioned between pairs of burners by utilizing a simple pneumaticvalve. In this preferred embodiment excess air is proportioned betweentwo burners with the use of control loops having only two moving parts,two constant volume pumps.

The invention is best understood by referring to the accompanyingdrawings wherein FIGURE 1 depicts the first embodiment of thisinvention. A burner 1 having a fuel inlet line 2, combustion air inletline 3, and frame 4 is controlled by a stream of control fluid suppliedby constant volume pump 5 via conduit 6 to an analog controller 7. Thefluid conduit 6 is placed in a position relative to the flame 4 so thatit is heated exclusively by that flame and not by the flames of theburners. Thus, the increase in energy of the control fluid in conduit 6is a function of the flame temperature of flame 4 and not of any otherflame. In addition, the fluid conduit should be insulated downstreamfrom the flame so that it does not lose a substantial portion of theenergy received from the flame. The analog controller 7 continuously orintermittently obtains a signalrepresenting the flow rate of combustionair from orifice 8 and changes the flow of combustion air by adjustingvalve 9 in rei spouse to a change in energy of the heated control fluid.Thus, in response to an increase in pressure or volume of the controlfluid resulting from an increase in flame temperature the pneumaticanalog controller would adjust valve 9. If the flame temperature goesup, the analog controller will increase the flow of air to the burner.

Instead of controlling the flow of combustion air to maintain theoptimum excess air to a burner, the fuel flow may be controlled. Thus,if conduit 3 were the fuel line and conduit 2 the combustion air line,the pneumatic analog controller 7 would increase the flow of fuel inresponse to a decrease in energy of the control stream which indicatesthat the flame temperature went below the desired range.

In each embodiment of the present invention, the control fluid pumpsshould provide a constant volume of control fluid to the system.Mre0ver, the temperature of the incoming control fluid should bemaintained s11bstantially constant so that a constant mole flow rate ofcontrol fluid is achieved.

FIGURE 2a illustrates the preferred embodiment of this invention whereina constant volume of combustion air is proportioned between two burners.A constant supply of combustion air is fed to burners 10 and 11 viaconduit 12. The combustion air from conduit 12 is divided in a pneumaticvalve described in FIGURES 2b and 2c which are hereinafter described. Aconstant mole rate of control fluid is pumped by constant volume pump 13into conduit 14 which passes in close proximity to flame 15 of burner10. Likewise, control fluid is provided by pump 18 to conduit 16 whichpasses in close proximity to flame 17. Conduits 14 and 16 must be placedso that they are heated exclusively by flames 15 and 17, respectively.In this system the mole flow rate of control fluid entering conduit 14must be the same as that entering 16, and the control fluid conduitsmust be placed so that there is no substantial disparity between theenergy increases of the two streams when their respective burn ers haveflames at the same temperature. Conduit 14 leads to port R in thepneumatic valve, whereas conduit 16 leads to port L on the pneumaticvalve. Thus, in the situation where the flame temperature of flame 15 isthe same as that of flame 17, the momentum of the control fluid to portL will be the same as the momentum of the control fluid to port R. Inthis situation the flow of combustion air coming through conduit 12 willbe equally divided between the two burners. If, however, the flow offuel via conduit 19 to burner 11 increases or if there is nozzle Wear atburner 11 causing an increased fuel flow to burner 11, which increasesthe flame temperature, the increased temperature of flame 17 will heatthe control fluid in conduit 16 to a higher temperature than that of thecontrol fluid in conduit 14. Therefore, the stream of fluid to port Lwill have greater momentum than that of the stream of fluid to port R.In this situation, the flow of combustion air is diverted so that anincreased amount goes to flame 17 thereby decreasing the flametemperature. The decrease in flow of combustion air to flame 15 willresult in an increase in flame temperature thereof. Therefore, thecontrol fluid in conduit 14 will be heated and the flow of momentumfluid to port R will be increased, thus tending to divert a portion ofthe combustion air back to flame 15. The system will soon balance outand the result will be that burners 10 and 11 will be operating atdifferent levels of excess air and flames 15 and 17 will be at differentflame temperatures but the difference will be slight and each burnerwill be operating within the optimum range of excess air rather thanhaving one burner operate out of the range and one in the middle of therange.

For violent upsets the flow of combustion air in conduit 12 may not besuflicient to permit the balancing of the burners within the optimumrange. Therefore, periodically a flue gas analysis should be performedto determine whether the burners are operating outside of the range.Even with more elaborate and complicated control systems, a flue gasanalysis must be performed periodically. In the simple system describedherein, the need for obtaining a flue gas analysis at frequent intervalsis obviated. The proportioning system of this invention will keep eachpair of burners operating within the optimum range as long as suflicientcombustion air is provided. It must be stressed, however, that this isnot a primary control technique and that the fuel air ratio mustperiodically be adjusted if the flue gas analysis so indicates.

A better understanding of the pneumatic valve used in this system isachieved by examining FIGURE 2b wherein is shown a valve for splittingthe combustion air in con-duit 12. The control fluid in conduit 16enters port L and the heated control fluid in conduit 14 enters port Ras is shown in the drawing. When the momentums of the control streamsare equal, the supply of combustion air is evenly distributed to bothburners (see FIGURE 2b). However, in the instance where the flametemperature of flame 17 increases, the energy of the control fluid inconduit 16 increases, thereby diverting a greater portion of thecombustion air to burner 11 (see FIGURE 2c). It is thus seen that theproportion of the total amount of combustion air supplied via conduit 12that is sent to burner 11 varies directly with the flame temperature offlame 17.

The amount of control fluid that is needed in conduits 14 and 16relative to the amount of combustion air supplied in conduit 5 (seeFIGURE 2a) is very small. If the control fluid is air, the fluid wouldhave to be supplied within the range of from 1 to 5 mole percent of thecombustion air and preferably only 2 mole percent. Larger amounts ofcontrol fluid can be used, but are not usually needed.

The Y-shaped pneumatic control valve depicted in FIGURES 2b and 2c hasheretofore been used to switch the flow of liquids, gases, slurries, orpneumatically-conveyed solids from one outlet to another in largepipelines. Such valves have not been used for the purposes describedherein and additionally, it should be noted that the control fluid whichinitiates changes in a manipulated variable, combustion air flow rate,or as hereinafter described, fuel rate, does so in direct response to achange in flame temperature which is an indirect measurement of acontrolled variable, to wit, the amount of excess air.

The manipulated variable in the system described in FIGURES 2a, 2b, and2c is the flow rate of the combustion air. However, since the pneumaticvalves can be used to proportion liquid streams, the manipulatedvariable can be the fuel flow. Referring to FIGURE 3, one sees twoburners, 20 and 23, fed by fuel via conduit and combustion air viaconduits 27 and 29. Control fluid is heated by flames 28 and 30* viapump 39, conduit 31, and pump 33, conduit 35, respectively. Where thefuel rate is the manipulated variable, however, the control fluid inconduit 31 is fed to port M, whereas the control fluid in conduit 35 isfed to port S so that the flow rate of fuel to a burner is made to varyinversely with the flame temperature of that burner.

A flow diagram for bustion air to eight burnersis illustrated in FIGURE4. The pneumatic valves shown therein are the same as the one describedin FIGURE 2b. Pneumatic valve 40 proportions the combustion air enteringthe furnace via conduit 41 between the two banks of burners. In thefirst bank of burners pneumatic valve 43 proportions air be tweenburners 45 and 47 in response to control fluid supplied via conduits 49and 51 by the constant volume pumps 53 and 55. Pneumatic valve 57proportions a supply of combustion air between burners 59 and 61 in response to control fluid furnished via lines 63 and 65 by proportioningthe supply of comcontrol pumps 67 and 69. Pneumatic valve 71 proportionscombustion air between pneumatic valves 43 and 57 in response to signalsvia conduits 73 and 75 wherein control fluid is supplied by pumps 77 and79.

Likewise, in the second bank of burners, the flow of combustion air toburners 81 and 83 is proportioned by pneumatic valve 85 in response tocontrol fluid supplied via conduits 87 and 89 by pumps 91 and 93. Thecombustion air to burners 95 and 97 is proportioned by pneumatic valve99 in response to control fluid signals received via conduits 101 and103 and supplied by pumps 105 and 107. In response to changes in energyof the control streams supplied by conduits 111 and 113, pneumatic valve115 proportions the combustion air between pneumatic valves 85 and 99.If there is a discrepancy between the average flame temperature of theburners in the first bank and the second bank, pneumatic valve 40adjusts the sytem to send more combustion air to the bank of burnershaving the higher average flame temperature in response to a differencein signals received via conduits and 123 wherein control fluid is passedthrough the flames of burners 61 and 97 as a result of the pumpingaction of pumps 125 and 127. In this fashion, a fixed amount ofcombustion air supplied via conduit 41 is proportioned between theburners in order to keep each burner operating within the optimum rangeof excess air. If the flue gas analysis indicates that the burners arenot operating within the optimum range with the use of this system, thetotal amount of combustion air can be changed or the fuel rates can bemodified. This system obviates the need for any major readjustmentsresulting from minor disturbances in the system.

This invention has been described with a certain degree ofparticularity. Certain deviations therefrom can be made withoutdeparting from the scope of the invention.

What is claimed is:

1. A control system for regulating the flow of combustion air to aburner comprising, in combination, a burner, a Y-shaped flow divertervalve for splitting a main stream of burner combustion air into twosecondary streams of predetermined proportion, said valve including acontrol port for the application of a flow proportioning control fluidthereto, means for supplying a constant volume flow of control fluid,conduit means connecting said control fluid supply means to said controlport, said conduit means passing adjacent the flame produced by saidburner to provide that the control fluid pumped through said conduit beexclusively heated by the flame of said burner whereby the energy of theheated control fluid changes in proportion to a change in temperature ofthe flame, said control fluid being directly effective to alter theratio of combustion air flowing in said secondary streams of saiddiverter valve.

2. A method of proportioning a constant volume stream of combustion airto two burners to obtain an amount of excess air at each burner withinthe desired range comprising dividing a combustion air stream to supplycombustion air to a first burner and a second burner, heating a firststream of control fluid with the flame of the first burner, heating asecond stream of control fluid with the flame of the second burner,whereby the energy of said first stream and said second stream ischanged in direct proportion to a change in flame temperature of saidfirst burner and second burner respectively; correcting, in response toa change in energy of either control stream, the flow of combustion airto said first burner and second burner in amounts directly proportionalto the change in energy of the first and second control streamsrespectively.

3. A method according to claim 2 wherein said first heated controlstream in injected into said combustion air stream at a positiondiametrically opposed to that at which the second heated control streamis injected into said combustion air and wherein both control streamsare injected into said combustion air stream at essentially right anglesthereto.

4. A method of proportioning a volume of fuel to two burners to ensurethat the burners operate within the optimum range of excess aircomprising dividing a stream of fuel to supply fuel to a first burnerand a second burner, heating a first stream of control fluid with theflame of the first burner, heating a second stream of control fluid withthe flame of the second burner, whereby the energy of said first streamand the energy of said second stream is changed in direct proportion toa change in flame temperature of said first burner and said secondburner respectively; changing, in response to a change in energy ofeither control stream, the flow rate of fuel to said first burner andsecond burner in amounts inversely proportional to the change in energyof the first and second control streams respectively.

5. A method according to claim 4 wherein said first heated controlstream is injected into said fuel stream at a position diametricallyopposed to that at which the second heated control stream is injectedinto said fuel stream and wherein both control streams are injected intosaid fuel stream at essentially right angles thereto.

6. An apparatus for proportioning a fixed supply of combustion airbetween the flames of two burners comprising: a pneumatic valve throughwhich said combustion air is channeled to a first burner and a secondburner; 21 first fluid conduit positioned from said first burner toensure that the control fluid to be pumped through said first fluidconduit is exclusively heated by the flame of said first burner, saidfirst fluid conduit being attached to said pneumatic valve andpositioned relative thereto to ensure that the direction of the flow ofthe control fluid entering said valve is at a right angle to thedirection of the flow of combustion air flowing through said valve; asecond fluid conduit positioned from said second burner to provide thatthe control fluid to be pumped through said second fluid conduit beexclusively heated by the flame of said second burner, said second fluidconduit being attached to said pneumatic valve at a positiondiametrically opposite of the position at which said first fluid conduitis attached to said pneumatic valve and said second fluid conduit beingpositioned relative to said pneumatic valve to ensure that the directionof the flow of the control fluid entering said valve is at a right angleto the flow of combustion air flowing therethrough.

References Cited UNITED STATES PATENTS 1,455,633 6/ 1923 Lundgard.1,630,318 5/1927 Tate. 2,848,868 8/1958 Jensen. 3,083,574 4/1963Messerly.

FOREIGN PATENTS 90,639 10/ 1937 Sweden.

EDWARD J. MICHAEL, Primary Examiner.

